In general, the reciprocating pump of single plunger type generates a greater pulsation of discharged fluid because of its discharge volume of zero upon suction stroke. In order to avoid such pulsation of the discharged fluid, a pump for controlling the total discharge volume to a constant volume has been proposed, in which two plungers of the same diameter are arranged in such a manner that the reciprocating phase of each plunger is shifted by 1/2 cycle from each other. In such type of the pump, one or two cams having a special curved face are used for providing a constant total volume discharged by the plungers, as described hereinbefore, whereby the combined volume discharged by pumping action of the two plungers may be theoretically free of pulsation. An embodiment of such type of the pulsation-free reciprocating plunger pump is illustrated in FIG. 1, wherein the two plungers 10, 10 are reciprocated by means of the cam driving for providing a combined volume discharged through pumping action of these plungers. The pair of plungers 10, 10 are arranged in parallel, with which is engaged a single rotary cam 12 having a cam surface at its one end. The cam surface is machined so as to provide a curved surface for keeping the constant combined volume discharged by the pumping action of the two plungers, which move along the periphery of the cam surface. In FIG. 1, each cross-head 14, 14 for transmitting a displacement of the rotary cam 12 to each plunger 10, 10 is provided with each of cam followers 16, 16 which is pressed against the cam surface by each spring 18, 18 arranged around the cross-head 14, 14. Further, the rotary cam 12 is provided at its center of opposite surface with a rotary shaft 20 to form a cantilever structure. The plungers 10, 10 are inserted through glad seals 24, 24 into respective pump chambers 22, 22, from which are derived discharge pipings 26, 26 which in turn are joined together at a point A.
Thus, the pump as illustrated in FIG. 1 may obtain, from its constructional view points, the pulsation-free property as shown in FIG. 2(a) and provide the pulsation-free combined discharge volume. However, it is difficult to completely avoid the pulsation of the combined discharge volume due to the following factors:
(1) A rounded portion must be provided at an inflexion point of the cam upon machining. PA1 (2) Velocity characteristics of the cam or the plunger can be varied due to the machining error of the cam. PA1 (3) Accurate and strick machining and arrangement of the cam must be effected for shifting the phase of two plungers in 180.degree. from each other. PA1 (4) A slight back-flow occurs through a check valve connected to a line leading to the pump. PA1 (5) A pumping liquid has its intrinsic compression. PA1 (6) A pumping liquid contains air bubbles and particles. PA1 [I] A method in which a velocity curve of cam is designed so as to increase a velocity of plunger only in a given range of a beginning delivery stroke of the plunger, thereby to correct the reduction of discharge volume due to the compression of liquid and the back-flow through the valve. PA1 [II] A method in which discharging pressures are determined for several strokes of the plunger to obtain an average valve, to which is approached a discharging pressure of subsequent stroke.
As a measure for correcting the factors (4) and (5) listed hereinabove, which vary rather regularly, the following two methods have been proposed:
However, the former method has such the disadvantages that the pulsation nevertheless increases in a zone having a relative low discharging pressure because of less compression or less back-flow of the liquid, and that the effect of reducing the pulsation through correction decreases as the pressure applied by a corrected curve of the cam is deviated from the pressure actually employed. On the other hand, the latter method has such the disadvantage that the average discharging pressure determined for several strokes can not be followed up the variation in the actual discharging pressure. Furthermore, the methods I and II have a common disadvantage in that an irrefular pulsation incapable of being responded appears so that the variation in the actual discharge volume depends largely on the cam characteristics.
Now it has been found out that the problems and disadvantages described hereinbefore may be solved all at once by the arrangement in that a cam for driving two plungers is connected to a driving motor, to which is in turn connected a rotation controlling circuit, while a pressure detector is provided for detecting a pressure of combined discharge volume and that a variable pressure signal in the pressure detector is fed to the rotation controlling circuit as a corrected signal.